The present invention relates to a solid type sensor for a nitrogen oxide gas, and more specifically to a nitrogen oxide gas sensor suitable for measuring NO.sub.x discharged from general combustion system and NO.sub.x in indoor environments, and particularly for sensing NO.sub.x in the exhaust gas of automobiles, where the gas temperature may reach quite high degree.
A gas sensor of the system of forming a sensing electrode and a counter electrode thereof on a zirconia solid electrolyte substrate and sensing the concentration of NO.sub.x by measuring the potential difference between the electrodes has already been reported. For example, gas sensors described in Japanese Patent Laid-Open Publication No. Hei 7-198671 and Japanese Patent Laid-Open Publication No. Hei 8-43346 each has a construction that a sensing electrode made up of a metal oxide such as CdMn.sub.2 O.sub.4 or NiCr.sub.2 O.sub.4 and a Pt counter electrode are formed on a zirconia solid electrolyte substrate as an oxygen ion conductor and it can be said, therefore, that these gas sensors have a heat resistance capable of being used in a high-temperature atmosphere.
On the other hand, as a sensing electrode having a sufficient heat resistance in a high-temperature atmosphere such as in an exhaust gas of automobiles, etc., even a noble metal electrode can be expected to be used. In this point, a Pt electrode has been already used as an electrode in a .lambda. oxygen sensor of automobiles and a linear air-fuel oxygen sensor and the high reliance in the practically used has been already proved. The noble metal electrode has many merits of a chemical stability thereof, the easiness of the preparation thereof, capability of expecting the reduction of the impedance of the electrode, etc. The examples of the sensor relating to a NO.sub.x gas using a noble metal sensing electrode on a zirconia solid electrolyte substrate cited in Japanese Patent Laid-Open Publication No. Hei 8-271476 are as follows.
One of them is disclosed in U.S. Pat. No. 4,199,425, which illustrates a sensor prepared by forming an alumina overcoat layer impregnated with Rh for imparting a NO.sub.x sensing to a concentration-cell type oxygen sensor (.lambda. sensor) for automobiles. However, in this sensor structure, the role of the overcoat layer impregnated with Rh is a NO.sub.x decomposition catalyst layer and it is clear that oxygen itself formed by the decomposition of NO.sub.x is sensed by the Pt sensing electrode.
Another one is shown in Japanese Patent Laid-Open Publication No. Sho 59-91358, which discloses a sensor prepared by forming an electrode made up of a noble metal such as Pt, Rh, Pd, Au, etc., and a sensing electrode prepared by laminating or applying an N.sub.2 O decomposition catalyst such as Co.sub.3 O.sub.4 on the above-described electrode on a zirconia solid electrolyte substrate, and in which the potential difference between the electrodes is measured. However, in the case of measuring NO.sub.x in the exhaust gas from automobile engines, the target gases are NO and NO.sub.2 and thus by the sensor for N.sub.2 O, the sum of NO and NO.sub.2 gases cannot be measured. Furthermore, the potential difference as sensor outputs to a low-concentration gas is very small and in the middle concentration range (several 1000 ppm or lower) of the real exhaust gas, the potential difference is almost same as zero.
As described above, although a noble metal sensing electrode is used in a concentration-cell type NO.sub.x sensor, the role thereof is simply as a NO.sub.x decomposition catalyst or only functions as a current collector of collecting electron charges generated in the decomposition reaction by the catalyst layer. Furthermore, as mentioned in Japanese Patent Laid-Open Publication No. Hei 8-271476, it is the present state that in the NO.sub.x sensor using a conventional noble metal sensing electrode, the potential difference as sensor outputs is small, the dependence on the oxygen concentration in a detection gas atmosphere is strong, and further the sensor can only be operated and sensing with decomposing NO.sub.x.
As described above, in the potential difference-type NO.sub.x sensor using an oxide electrode, a high sensitivity is obtained but the resistance of the sensing electrode is high and thus a current collector must be formed in the sensing electrode to reduce the electrode area.
On the other hand, in the noble metal electrode which is a good electric conductor as an electrode material, there is no such an electrode which measures the NO.sub.x potential difference as it is. The noble metal electrode can have a little sensitivity only to N.sub.2 O (laughing gas). Furthermore, in the case of noble metal electrode, it is necessary to control the O.sub.2 concentration correctly because the potential difference depends on oxygen partial pressure.